A: Angela Palmer, About’s acne guide, lays out the following plan: Take out everything in your cabinet and look through it; check for expiration dates; if these dates are nowhere to be found, check the product’s consistency, color and smell; donate or give away any unwanted, unopened over-the-counter products; and use a household cleaner on your shelf and a damp cloth to wipe your products.

Nothing lasts forever, especially when it comes to cosmetics and skincare. Although you may hate to say goodbye to a favorite cleanser, cream or concealer — especially if the product was pricey — using a product after it’s expired can actually harm your skin. Unfortunately, knowing when to pitch products isn’t always so straightforward. Oftentimes, your beauty and skincare products don’t have an expiration date or when there is one, it’s simply a suggested use-by date. But the actual shelf life can depend on when products are opened and how they’re stored. Here are key pointers on prolonging your products along with guidelines on when it’s time to let them go.

Extending shelf life

You can often safely store unopened cosmetics for a long time in a dark, cool place, such as a drawer or closet. However, once a product has been opened and used, oxidation begins and bacteria can spread.

To protect your products from going bad, follow these general tips:

Wash your hands thoroughly before using any product.

Avoiding putting fingers directly into a jar or bottle. Instead, use a clean cotton swab to remove the product.

Opt for products that come in pumps or jars, which prevents air from getting into the product.

Close jar and container lids tightly between uses.

To minimize exposure to water and moisture, don’t leave skincare products in the shower. Instead, store them in the medicine cabinet or a closet, away from direct heat and light.

Keep makeup out of the bathroom. Bathrooms can become very humid — the perfect breeding ground for bacteria, mold or yeast.

Don’t leave makeup in the car. It can spoil or melt from the heat and sunlight.

Don’t share your products.

Throw away eye and lip products if you’ve recently had an eye infection or a cold sore — bacteria can live on the product’s surface.

Sharpen eye and lip pencils after every use.

Keep tools such as makeup brushes and pencil sharpeners fresh and bacteria-free by cleaning them frequently.

Expiration guidelines for makeup

Mascara: Three to six months

Mascara is one of the most volatile products in any beauty kit. If mascara smells or changes texture, throw it out. To avoid a bacterial invasion, refrain from pumping the wand to pick up more color — this only forces air into the tube and dries out the mascara. For a better way to evenly coat the wand, swipe it in a circular motion slowly around the inside of the tube.

Eye liner:Three months (liquid or gel liner) to two years (pencil)

Like mascara, liquid or gel eye liner is vulnerable to bacteria and susceptible to drying out if not stored properly. Pencil eye liner normally lasts longer than its liquid counterparts, but won’t last forever.

Lip liner: Two to three years

Lip liner pencils share similar characteristics with pencil eye liners and require similar care, which includes sharpening frequently and keeping the cap closed tightly when not in use.

Eye shadow: Six months (cream) to two years (powder)

Although eye shadow tends to last longer than other types of makeup, you should still watch for signs of wear. Toss eye shadow when pigments change or if you notice waxy buildup. To maximize eye shadow life, invest in fewer colors and buy smaller sizes. Also, if you add water to powder shadow for darker, more intense color, don’t keep it longer than six months.

Foundation:Six to 12 months

Whether water-based or oil-based, foundation can also become a breeding ground for bacteria. With age and exposure to light or heat, the consistency and texture of foundation degrade. Foundation may also develop a foul smell or become discolored over time. These are definite signs that it’s time to ditch your foundation.

Concealer: Six to 12 months

Often formulated with oil or hydrating agents such as jojoba oil or shea butter, concealers usually have a smooth, creamy texture for easy blending. Watch for signs of texture change: If concealer separates, dries out, is cakey or smells of mildew, pitch it.

Powder: Six months to two years

It can be tough to tell when to pitch your powder. It generally won’t smell or change color or texture like cream, but powder can go bad. Even though powders are dry and bacteria can’t thrive without water, some formulas have trace amounts of water or hydrators, like aloe, jojoba oil or shea butter.

And you can unknowingly transfer bacteria from your face to your powder with a powder brush. To minimize this risk, wash powder brushes regularly with an antibacterial brush cleaner. Toss the powder if it changes texture and becomes grainy and hard to blend, or crumbles easily.

Lipstick: Two years

Although lipstick can last quite a while, remember that it contains water, moisturizers and hydrators and is easily infiltrated by bacteria. Also, it can eventually dry out and become difficult to apply. Toss lipstick when it becomes dry, waxy or smells different — rancid lipstick often starts to smell like crayons.

Expiration guidelines for skincare

Cleanser: One year

Be aware that certain active ingredients (glycolic acid and retinol) might become more potent if stored for long periods of time, potentially causing an allergic reaction.

Serum: Six to 12 months

Serums are vulnerable to air, heat and light. If possible, opt for a serum in a pump bottle, which will maintain the integrity of the product longer than a jar. Also, many serums contain unstable ingredients to combat fine lines and wrinkles. Vitamin C, for instance, can spoil quickly when exposed to air. Once oxidized, vitamin C (and all) serums will darken or yellow, and might even smell rancid — clear indicators that it’s time to toss.

Moisturizer: One year

Moisturizer will last longest in a tube or bottle with a pump dispenser as opposed to a jar. Pay attention to expiration dates; using an expired moisturizer over time can give you a rash or infection, especially if your moisturizer contains alpha hydroxy acids. Finally, no matter how much time has passed, toss your moisturizer if you notice a change in consistency or smell.

Sunscreen: One year

While many of us use sunscreen year-round, summer is undoubtedly when most of us buy and use this vital product. For some people, three months is enough time to use an entire bottle, while others tend to keep old sunscreen well past the season.

Like all creams, sunscreen is susceptible to oxidation. Exposing the cream to air causes water to evaporate and the product to break down, which can ultimately make the formula unstable and ineffective. Store sunscreen in a dark, cool place, and when on the beach, replace the cap and avoid leaving the sunscreen in direct sunlight.

Acne treatments: Two years

Over-the-counter acne-fighting gels and creams can last for several years, but they might expire a lot sooner if the active ingredients deteriorate. Any change in smell, color or consistency is a sign that the product has reached its peak.

Plus, acne treatments with certain ingredients — like benzoyl peroxide — will sometimes sport an expiration date, making it easy to figure out when to replace. As with serums and cleansers, store acne creams away from direct heat and light to prolong shelf life.

A note on natural products

Many consumers prefer to use natural beauty and skincare products. However, keep in mind that many of these products contain ingredients that spoil faster, or use different types of preservatives than traditional products. This means that your product might have a shorter shelf life than you anticipated. The rule of thumb? Watch for changes in smell, color and consistency. You can also contact the company for recommended expiration dates.

The bottom line

Above all, remember that these are general guidelines, not set rules. We can’t underscore this enough: Any change in the smell, color or consistency of your products likely means it’s time to toss. Although it’s tempting to keep favorite products for years, you may be doing your skin more harm than good. Spoiled skincare and beauty products can cause irritation, allergic reactions and even infection. Maintaining healthy skin means updating your products on a regular basis.